Process and apparatus for producing metal sheets of better flatness

ABSTRACT

A process of cold rolling sheets in strip form under tension, preferably on a beam backed cluster mill while automatically controlling roll pressure distribution across the strip, so that strip tension is uniform all the way across the strip. Static air pressure is applied against the strip to deflect away from the pass line. Any unevenness in strip tension results in an uneven deflection, which in turn produces variations in the rate of flow of air escaping through clearances in the deflecting instrumentalities which affects their internal air pressure. Those differences are employed directly to operate the roll pressure distribution in the mill. &#39;&#39;&#39;&#39;On&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;off&#39;&#39;&#39;&#39; timing of the control mechanism avoids complex instrumentation and provides maintenance-free operation. When tension at any spot transversely of the strip drops to zero thus heralding the formation of a cobble, the apparatus actuates a wedge which stops the strip instantly, leaving no material from which to form a cobble. Pulsating air pressure which is inherently associated with the use of volumetric blowers is further utilized to produce high frequency rythmic variations in strip tension, preferably opposed in phase as to the strip segments ahead and behind the work rolls. This reduces the roll separating force and permits substantially heavier reductions per pass. Apparatus for performing the process is also disclosed.

United States Patent [191 Sendzimir 1 51 Nov. 25, 1975 TadeuszSendzimir, PO. Box 1350, Waterbury, Conn. 06720 [22] Filed: June 13,1974 [211 Appl. No.: 479,032

[76] lnventor:

[52] U.S. Cl 72/4; 72/10 [51] Int. Cl. B21B 33/00; 8218 37/06 [58] Fieldof Search 72/6, 8, 9,10,11,12, 72/16, 20, 3, 4, 21

[56] References Cited UNITED STATES PATENTS 2,194,212 3/1940 Sendzimir72/242 X 2,479,974 8/1949 Sendzimirm, 72/249 2,728,223 12/1955 Herrman73/144 3,020,788 2/1962 Peters 72/234 X 3,315,506 4/1967 Schneider. 72/93,318,129 5/1967 Gross 72/199 3,402,603 9/1968 Hollister et al. 73/1593,496,744 2/1970 Mizuno et al.... 72/12 3,499,306 3/1970 Pearson 72/173,552,161 1/1971 Garbe et al. 72/4 FOREIGN PATENTS OR APPLICATIONS946,661 l/1964 United Kingdom 72/21 Primary ExaminerMilton S. MehrAttorney, Agent, or FirmMelville, Strasser, Foster & Hoffman [57]ABSTRACT A process of cold rolling sheets in strip form under tension,preferably on a beam backed cluster mill while automatically controllingroll pressure distribution across the strip, so that strip tension isuniform all the way across the strip. Static air pressure is appliedagainst the strip to deflect away from the pass line. Any unevenness instrip tension results in an uneven deflection, which in turn producesvariations in the rate of flow of air escaping through clearances in thedeflecting instrumentalities which affects their internal air pressure.Those differences are employed directly to operate the roll pressuredistribution in the mill. On and off" timing of the control mechanismavoids complex instrumentation and provides maintenance-free operation.

When tension at any spot transversely of the strip drops to zero thusheralding the formation of a cobble, the apparatus actuates a wedgewhich stops the strip instantly, leaving no material from which to forma cobble.

Pulsating air pressure which is inherently associated with the use ofvolumetric blowers is further utilized to produce high frequency rythmicvariations in strip tension, preferably opposed in phase as to the stripsegments ahead and behind the work rolls. This reduces the rollseparating force and permits substantially heavier reductions per pass.

Apparatus for performing the process is also disclosed.

7 Claims, 6 Drawing Figures US. Patent N0v.25, 1975 Sheet 1 of23,921,425

FIGJ

US, Patent Nov. 25, 1975 Sheet20f2 3,921,425

PROCESS AND APPARATUS FOR PRODUCING METAL SHEETS OF BETTER FLATNESSBRIEF SUMMARY OF THE INVENTION Beam backed multi-roll mills for rollingflat articles such as are shown, for example, in US. Pat. No. 2,479,974,provide for instantaneous change of the roll profile by controlling theheight of the spaced roll supports. This produces an increase ordecrease in work roll pressure at any point across the face of the rolland thus makes it possible to produce a flat strip of raw ma terial ofnot quite uniform thickness and temper.

Typical flatness defects such as edge waves and pockets result from toogreat a roll pressure applied in certain areas, and flatness can becorrected by suitably adjusting the corresponding roll supports. Since atension strip appears flat to the eye during rolling, apparatus has beenprovided in the past to measure the distribution of tension across thestrip during rolling. usuallyby exerting a deflecting force against thestrip and placing proximity meters at various points across the strip tomeasure the deflection. A high deflection indicates low tension.

Such proximity meters of sufficient accuracy and small time lag areusually complex electronic devices; and an attempt to use theindications of such meters automatically to operate the controls ofeffective height of the spaced roll supports, leads to apparatus ofgreat complexity and greatly increases the maintenance difficultiesunder mill conditions.

The present invention also involves the use of uniform pressure appliednormally to the strip surface, preferably at points corresponding to thespaced roll supports, but in such a manner that the deflectinginstrumentality functions at the same time as a meter of strip tensionat that spot and also as an automatic actuator of the effective heightof that particular spaced roll support. Another benefit flowing from thepresent invention provides an improvement in the rolling action of themill itself as will become clear from the description hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING FIG. 1 is a sideelevation partly in section of one em bodiment of the present invention.

FIG. 2 is a side elevation partly in cross section taken on the line2--2 of FIG. 3, showing the invention applied to a simplified mill.

FIG. 3 is a cross sectional view taken on the line 3-3 of FIG. 2.

FIG. 4 is a schematic view of certain details of FIG. 2.

FIG. 5 is an enlarged detailed view ofone ofthe pressure pots and itsassociated gauge system.

FIG. 6 is an elevational view of another embodiment of the invention.

DETAILED DESCRIPTION Reference may be had to the Sendzimir US. Pat. No.2,479,974 for a detailed description of a typical beam backed multi-rollmill having means to adjust the roll profile. In FIG. 1 the presentinvention is shown in connection with a so-called ZO-Hi" mill while inFIG. 3 and FIG. 6 it is shown in connection with a so-called 6-Hi mill.It should be understood that the 6-Hi mill has been shown for simplicityand ease of understanding; but the same principles may be applied to the2 20-Hi or other cluster mill in which case the screwdown mechanismshown in FIG. 3 is applied to the two central ones of the beam backedbacking rolls.

Referring to FIG. I, a strip 1 is shown being cold rolled in a mill 2with strip tension being applied by the unwinder 3. The strip passesfrom the unwinder over a deflector roll 4 and thence horizontally intothe mill 2.

For ease of understanding, reference will now be made to FIG. 2 and 3showing the 6-Hi" mill. In this mill, spaced supports 5 are provided forthe shaft 9 which carries the backing bearings 8. The supports 5 rest introughs machined into the housing beam 2 for rigid support. Eccentrics 6are disposed inside the supports 5 and are free to rotate upon the shaft9 and are individually rotated by pinions which are integral with theeccentrics and which engage racks 7. The position of the racks 7 is inturn controlled by the worm nut and screw 7 and driven by motors 7"which have worms mounted on their shafts to turn the worm nuts andthereby raise or lower the racks 7. It will be clear that as the racksmove up or down they engage the opposed gear seqments of the eccentrics6 and by rotating the eccentrics 6 adjust the vertical position of thebacking bearings 8 and thus the vertical position of the work roll whichis supported thereby.

In the drawings, the screwdown mechanism has been shown as applied tothe lower set of rolls whereas in actual practice. the lower set ofrolls are adjustable only for opening the roll bite for threading andthe screwdown is generally applied to the upper rolls. However. forpurposes of describing the invention. it makes no difference whether thescrewdown is applied to the upper or lower rolls.

It will be clear now that by energizing one or more of the motors 7",the respective bearings 8 may be raised or lowered. whereby to controlroll pressure in the vicinity of each of the supports 5.

Control of the motors 7 is provided by means of apparatus which will nowbe described. Aerostatic pressure is applied against the strip 1preferably by means of pressure pots 10. These are shown schematicallyin plan view in FIG. 4 in relation to the positions of the spacedsupports 5 of FIG. 2. They are identified in the drawing by I to VIIinclusive. The several pressure pots are rigidly mounted on the beam 11(FIG. 5) which is provided with a wedge mechanism 35 at each end inorder to align the row of pots 10 with the mill passline. Each of thepots is provided with a cup lip 10 in threaded engagement with the cup10 so that the position of the lip may be adjusted accurately inrelation to the strip 1. Air is supplied in equal quantities by theindividual fixed displacement blowers 13 which are rotated by a commonmotor 13'. So long as the clearances between the several cup lips 10'and the strip I remain identical, the volume of air escaping throughthem will be the same and therefore the pressures in the pots 10 whichprovide for the deflection of the strip 1 will also be the same.

If now the central portion of the strip 1 is overrolled (the tradeexpression is it has a pocket), the tensioned strip still looksperfectly flat to the operator. but the thus locally dimished tensioncauses the clearances at the two corresponding pots III and IV to bemore than the clearances at the other pots, and therefore their staticpressure would be reduced.

Associated with each pressure pot there is a mercury column type ofpressure gauge 14 which is shown in more detail in FIG. 5. This deviceaccording to the present invention serves directly to control the motors7" and thus automatically to correct any local overor under-rolling soas to produce strips of good flatness. Air pressure from the pot 10 actsupon the mercury column 34 and any pressure fluctuations in the pot 10will move the mercury to or from the columns 34a and 34" to balance withatmospheric pressure. Adjustable height insulated electrodes l4. l4 and14a are provided for the respective mercury columns. These are disposedabove the mercury level and their contact with the mercury actuates thecorresponding motors 7" to raise or lower the respective screws 7 as thecase may be.

In order to prevent over-control or hunting. it is preferred to energizethe entire system (i.e.. admit electric current to it) for short periodsof time only by the use of any well known clock-type switch. An onperiod of a half second followed by a off period of ten seconds ispractical for medium speed mills. In this way. in the case discussedabove. if the half second correction has not changed the roll pressurein the areas controlled by the pots Ill and IV sufficiently. the nextten seconds will be spent measuring the new air pressures and if theyare still not within permissible limits. another half second correctionwill automatically follow. On faster mills the ten second lingeringperiods may be reduced. Also faster response motors. for example.hydraulic. would permit a reduction in the half second adjustmentperiods.

Air supplied by displacement blowers inherently produces a pulsatingstream of air and for this reason the needle valve 16 is provided tothrottle the flow of mercury in the pressure gauge 14 so that itindicates the average static pressure in each pot l0 and only theaverage pressure therefore controls the action.

Unexpectedly. however. a further advantage results from the highfrequency rythmic vibrations in strip tension in that it makes itpossible to take heavier pass reductions than when the tension issteady. This effect may be increased by tuning the frequencies andphases of the strip sectors on the up stream and down stream sides ofthe roll bite respectively so as to be synchronous but reversed inphase. so that the maximum tension ahead of the rolls corresponds tominimum tension behind the rolls. In order to produce these resonantvibrations, the deflector rolls 4 are mounted on slides and are providedwith motorized screws 17 to obtain the optimum length of vibratingstrip 1. This may be either a single wave form or a half wave with anode half-way or even shorter. Each wave leaves a slight butdistinguishable mark on the surface of the strip and these marks almostdisappear if they are small enough. If they have not disappeared. it maybe necessary to make a final pass as an iron-out pass taking only theregular reduction and without use of the vibration principle.

The vibrations described above have another beneficial effect in thatthe strip comes out flatter by itself which makes it easier to produce anearly perfect flatness ofthe strip. Yet another advantage is that ahigher front tension in relation to the back tension may be used sincethe vibrations yield maximum benefits of back tension and therefore theexcess of front tension can be used to assist the drive ofthe work rollsand thus take a bigger roll pass without strip slippage. This isimportant when small diameter work rolls are used.

An alternative embodiment of an apparatus useful in the pursuit of thepresent process is shown in FIG. 6. This is particularly useful forslower mills operating at 1000 feet per minute and below which are usedchiefly for expensive alloys rolled to light gauges where the operatorsskill in coping with metallurgical requirements to obtain the desiredquality is essential. Here the strip is subjected to a relatively muchlower static pressure but the pressure extends over a very large area.As seen in FIG. 6, a pressure box 20 is shown extending lengthwise ofthe strip between seals at the deflector roll 4 and the wiper roll 19which are very close to the work roll. Laterally. the pressure box 20extends between two lateral fences adjustable for strip width andproviding a small clearance between the upper lip of the fence and theedge of the strip through which air from the pressure box 20 is free toescape. The air is replenished and maintained at the required pressureby outside means such as one or more blowers 21. It may be observed thatthe pressure in the pots 10 in this embodiment is negative with respectto the box 20, but is still positive with respect to atmosphere.

Static pressure as low as a few ounces per square inch is usuallysufficient to deflect the strip a considerable amount. If the deflectionis only one percent of the length of the pressure chamber. flatnessdefects such as wavy edges or full centers or other irregularities canbe directly and visually detected if the light is played on the strip atthe correct angle for the operator. The operator therefore has theadvantage in being able to check the effect of the automatic flatnesscontrol and if he see fit. to take over manually in particularlystubborn cases.

While these relatively large deflections can be measured by varioussimple means. it is still preferred to provide a cross beam with thebank of pressure pots 10 just as described in connection with FIGS. 1 to5, except that air escapes from the pressure chamber 20 into the pot l0and from there through equalized calibrated outflow valves 22 into theatmosphere. Here again in areas where the clearance between the lip of apot l0 and the strip increases indicating stronger tension. the pressureinside the pot will decrease and vice versa. This change in pressure isthen utilized to directly activate the flatness control elements asdescribed in connection with FIGS. 1 to 5.

Another important advantage of the process and apparatus hereindisclosed is that it makes it possible to protect the mill against oneof the costliest accidents which can occur. known as a cobble. If astrip with an undetected side crack enters the roll bite, the portionsof the strip on both sides of the crack will, of course. have lost theirtension and the tension is then concentrated in the strip where thecrack is deepest. That in turn causes the tensionless portion of thestrip material entering the roll bite to fold over and form a so-calledharmonica which produces crushing pressures against that spot of thework roll and renders it useless in a fraction of a second. Furthermore,since the uncracked portion of the strip continues to be rolled. itdrags more of the harmonica into the roll bite and causes further damagewhich extends to the backing rolls and bearings.

Since the cause of each cobble is a tensionless portion of the strip andsince according to the present invention this condition can bedetermined instanta neously, although it may be only be milisecondsbefore the accident, an emergency mill stop or screwdown opening devicemay be provided for instantaneous operation. Referring back to FIG. 5,there is provided a 7 special leg 34a in the pressure gauge 14 and it isprovided with an electrode 14a which is adjusted to switch the currenton when the pressure in any pot drops to or near zero. The current tothe electrode 14a is on" continuously and is not periodicallyinterrupted as was the case with the electrodes 14' and 14'. This isbecause the cobble may occur at any time.

For faster protection of the mill it is preferred to stop the advance ofthe strip through the mill instantly and thus not to give the rolls anymaterial from which to form the harmonica. Thus, in FIG. 6 a gate isprovided through which the strip passes ahead of the rolls and whichconsists of a frame including a lower beam 24' having a wedge surfaceand an upper beam 24. Associated with the beam 24 is a wedge block 23operated by air cylinders 26 to push the wedge block 23 along the wedgesurface of the beam 24' into the above mentioned gate when the cylinders26 are actuated for example by a solenoid valve energized by theelectrode 14a of FIG. 5.

During rolling the strip 1 may pass through the gate with ampleclearance, but when a cracked strip approaches and any of the electrodes14a detects the loss of strip tension, the wedge 23 is substantiallyinstantaneously pushed into the gate over the tapered face of the beam24' along with the strip itself which inciden tally helps to push thebeam 23 tighter and stops the progress of the strip by gripping itbetween the members 23 and 24. The undersurface of the beam 24 and theupper surface of the wedge member 23 may be serrated to enhance thestopping action. The immediate sharp increase of tension resulting fromthe coaction of the block 23 and 24 causes the strip to break in therollbite and it is then only necessary to stop the inertia of the twowinders and the mill itself. During this operation, some strip maybecome unwound but damage to the mill is effectively avoided.

It will be understood that the disclosure herein is by way of exampleonly and that various modifications may be made without departing fromthe spirit of the invention. No limitation not expressly set forth inthe claims is intended and no such limitation should be implied.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In the tension rolling of metal strip in a mill having work rolls anda deflecting roll, wherein the strip passes from the deflecting roll tothe work rolls of a mill, the method of producing strip of improvedflatness while avoiding the danger of scratching the surface by contactwith any instrumentality, which includes the steps of applying a uniformdeflecting pressure to a plu' rality of areas of equal size and shapedisposed transversely of the strip, said areas being defined by theprojections, on the strip, of the perimeters of a number of pressurepots located close to, and parallel to, said strip, adjustably mountingsaid pots to maintain a fixed position relative to the strip, by causingequal volumes of air to pass between the boundaries of said areas andthe strip surface, thus producing a pressure differential between theinside and outside of said areas; the said pressure differential beinguniform for all areas so long as the strip deflections are uniform, butvarying when said deflections are not uniform, and adjusting the rollpressure transversely across the strip to restore the uniformity of saiddeflections when they vary.

2. The method of claim 1, wherein said air under pressure is caused topulsate, and including the steps of 6 deflecting the strip similarly onthe exit side of said mill by pulsating air pressure. and timing thefrequencies and phases of the portions of strip on the entrance and exitsides of the mill to be synchronous hut reversed in phase, wherebyheavier than normal pass reductions may be achieved.

3. The method of claim 1, including the step of causing a major decreasein tension at any of said locations. indicating the formation of acobble, to stop further motion of said strip substantiallyinstantaneously, to prevent damage to said mill.

4. In a beam-backed cluster mill having rotating backing elementsbetween each work roll and said backing beam, spaced across the workroll face. and adjustable supports between each two of said backingelements; a row of non-contacting pressure pots for deflecting thetensioned strip across its full width, means to supply air underpressure to said pots. the rims of said pots being disposed closelyadjacent the strip to be rolled, a pressure gauge for each pot, saidpressure gauges including electrical switching means for completing anelectrical circuit upon a predetermined increase and decrease inpressure at the related pot. said circuit including the related motormeans, said switching means comprising two interconnected columns ofmercury, and a vertically adjustable electrode for each column, wherebyupon the increase in pressure the mercury will rise in one of saidcolumns and make contact with the related electrode, and upon a decreasein pressure the mercury will rise in the other of said columns and makecontact with the related electrode.

5. In a beam-backed cluster mill having rotating backing elementsbetween each work roll and said backing beam, spaced across the workroll face, and adjustable supports between each two of said backingelements; a row of non-contacting pressure pots for deflecting thetensioned strip across its full width, means to supply air underpressure to said pots, the rims of said pots being disposed closelyadjacent the strip to be rolled, the air supply means being a pulsatingone, and like deflecting instrumentalities being similarly disposed onthe exit side of said mill, there being also a deflector roll on saidexit side, said deflector rolls being adjustable toward and away fromthe work rolls of the mill, whereby the frequency and phase of theportions of the strip on the entrance and exit sides of the mill may betuned to be synchronous but of opposite phase, to permit the achievementof heavier than normal pass reductions.

6. In a beam-backed cluster mill having rotating backing elementsbetween each work roll and said backing beam, spaced across the workroll face, and adjustable supports between each two of said backingelements; a row of non-contacting fluid pressure means for deflectingthe tensioned strip across its full width. means for controlling rollpressure across the strip to maintain uniform the resulting stripdeflections, parallel beams above and below the strip on the entranceside of the mill, one of said beams having a sloping surface, a wedgeblock arranged to move on said sloping surface, power means to move saidwedge block, cobble cause sensing means associated with each of saidfluid pressure means, and an operative connection between said cobblecause sensing means and said power means, whereby upon the occurrence ofa cobble causing event said wedge block, in cooperation with the opposedbeams, will instantaneously stop the strip and prevent said cobble fromdamaging the mill.

8 sioned strip across its full width. a roll of non-contacting pressurepots having rims adjustable disposed closely adjacent the strip to berolled. the pressure in said pots being negative with respect to thepressure in said pressure box but positive with respect to atmosphere,and means to detect pressure fluctuations in said pots due todifferences in the escape of air from said pressure box into saidpressure pots.

1. In the tension rolling of metal strip in a mill having work rolls anda deflecting roll, wherein the strip passes from the deflecting roll tothe work rolls of a mill, the method of producing strip of improvedflatness while avoiding the danger of scratching the surface by contactwith any instrumentality, which includes the steps of applying a uniformdeflecting pressure to a plurality of areas of equal size and shapedisposed transversely of the strip, said areas being defined by theprojections, on the strip, of the perimeters of a number of pressurepots located close to, and parallel to, said strip, adjustably mountingsaid pots to maintain a fixed position relative to the strip, by causingequal volumes of air to pass between the boundaries of said areas andthe strip surface, thus producing a pressure differential between theinside and outside of said areas; the said pressure differential beinguniform for all areas so long as the strip deflections are uniform, butvarying when said deflections are not uniform, and adjusting the rollpressure transversely across the strip to restore the uniformity of saiddeflections when they vary.
 2. The method of claim 1, wherein said airunder pressure is caused to pulsate, and including the steps ofdeflecting the strip similarly on the exit side of said mill Bypulsating air pressure, and timing the frequencies and phases of theportions of strip on the entrance and exit sides of the mill to besynchronous but reversed in phase, whereby heavier than normal passreductions may be achieved.
 3. The method of claim 1, including the stepof causing a major decrease in tension at any of said locations,indicating the formation of a cobble, to stop further motion of saidstrip substantially instantaneously, to prevent damage to said mill. 4.In a beam-backed cluster mill having rotating backing elements betweeneach work roll and said backing beam, spaced across the work roll face,and adjustable supports between each two of said backing elements; a rowof non-contacting pressure pots for deflecting the tensioned stripacross its full width, means to supply air under pressure to said pots,the rims of said pots being disposed closely adjacent the strip to berolled, a pressure gauge for each pot, said pressure gauges includingelectrical switching means for completing an electrical circuit upon apredetermined increase and decrease in pressure at the related pot, saidcircuit including the related motor means, said switching meanscomprising two interconnected columns of mercury, and a verticallyadjustable electrode for each column, whereby upon the increase inpressure the mercury will rise in one of said columns and make contactwith the related electrode, and upon a decrease in pressure the mercurywill rise in the other of said columns and make contact with the relatedelectrode.
 5. In a beam-backed cluster mill having rotating backingelements between each work roll and said backing beam, spaced across thework roll face, and adjustable supports between each two of said backingelements; a row of non-contacting pressure pots for deflecting thetensioned strip across its full width, means to supply air underpressure to said pots, the rims of said pots being disposed closelyadjacent the strip to be rolled, the air supply means being a pulsatingone, and like deflecting instrumentalities being similarly disposed onthe exit side of said mill, there being also a deflector roll on saidexit side, said deflector rolls being adjustable toward and away fromthe work rolls of the mill, whereby the frequency and phase of theportions of the strip on the entrance and exit sides of the mill may betuned to be synchronous but of opposite phase, to permit the achievementof heavier than normal pass reductions.
 6. In a beam-backed cluster millhaving rotating backing elements between each work roll and said backingbeam, spaced across the work roll face, and adjustable supports betweeneach two of said backing elements; a row of non-contacting fluidpressure means for deflecting the tensioned strip across its full width,means for controlling roll pressure across the strip to maintain uniformthe resulting strip deflections, parallel beams above and below thestrip on the entrance side of the mill, one of said beams having asloping surface, a wedge block arranged to move on said sloping surface,power means to move said wedge block, cobble cause sensing meansassociated with each of said fluid pressure means, and an operativeconnection between said cobble cause sensing means and said power means,whereby upon the occurrence of a cobble causing event said wedge block,in cooperation with the opposed beams, will instantaneously stop thestrip and prevent said cobble from damaging the mill.
 7. In abeam-backed cluster mill having work rolls and a deflector roll spaced asubstantial distance in advance of said work rolls, and having rotatingbacking elements between each work roll and said backing beam, spacedacross the roll face, and adjustable support between each two of saidbacking elements; a pressure box extending substantially over the entirewidth of the strip being rolled, and extending over the major portion ofthe length of the strip between said deflector roll and said work rolls,for deflecting the tenSioned strip across its full width, a roll ofnon-contacting pressure pots having rims adjustable disposed closelyadjacent the strip to be rolled, the pressure in said pots beingnegative with respect to the pressure in said pressure box but positivewith respect to atmosphere, and means to detect pressure fluctuations insaid pots due to differences in the escape of air from said pressure boxinto said pressure pots.